Oxygen dependent quenching of phosphorescence is a sensitive and robust method for measuring molecular oxygen, particularly in tissue in vivo. It has been demonstrated that 3D imaging of oxygen in tissue can be accomplished using near infrared light when near-infrared absorbing phosphorescent oxygen sensors are added to the blood. We propose to develop non-toxic optical nanosensors suitable for measuring and imaging oxygen in tissue microvascualture in vivo, ready for translation into clinics. We will synthesize bio-compatible oxygen nanosensors with near infrared absorption and emission and quenching constants optimized for measurements in tissue. The phosphors will be multi-layered, i.e. metal complexes of tetrabenzopoiphyrins, encapsulated inside dendrimers and coated with oligoethyleneglycols (PEG). The near-infrared absorbing luminescent metalloporphyrins will act as the actual sensors for oxygen. Dendrimers will be covalently linked to the porphyrins to form protective shells, tuning oxygen diffusion rates to the values optimal for tissue oxygen measurement. The dendritic shells will also help to protect the porphyrin core from interacting with blood components and will serve as scavengers for singlet oxygen. Porphyrin dendrimers will then be targeted for rapid excretion (via the kidney) by coating their external surfaces with PEGs. PEGs will isolate the porphyrin-dendrimers from interactions with other blood components. Thus, the sensor luminescence intensity and lifetime will become completely insensitive to species in the environment other than oxygen. Acute and long term toxicity of the nanosensors will be exhaustively tested in the newborn piglet model and mice model, respectively. The goal is to create a nanosensor(s) for oxygen that is suitable for clinical use, moving it to the point where it is ready for translation into the clinics i.e. ready for larger scale synthesis and rigorous toxicology studies preparatory to use in clinical trials.